Rocks of Noyang. 29 



Disregarding the impurities, this mineral maybe considered 

 as composed of nearly 9 per cent, of chlorite and 91 per 

 cent, of chloritoid.* 



The molecular proportions of the sesquioxide and pro- 

 toxide bases in this chloritoid do not agree well with those 

 of other chloritoids, the analyses of which I have been 

 enabled to examine and calculate ; "f yet the whole mineral, 

 including impurities, gives a formula which accords quite as 

 nearly with that of chloritoid as do those of some of the 

 analyses to which I refer. 



This raises a doubt whether those analyses were not also 

 of impure or mixed material. It is to be feared that in too 

 many cases new species of chloritic minerals have been estab- 

 lished by analysts on all too insufficient examination, and 

 that mineralogical science has been overburdened with names 

 that will ultimately have to be expunged. 



This Noyang mineral, being a mixture, has not even a 

 right to be called a mineral species. It is a mixture of 

 chloritic minerals, and I doubt whether, strictly speaking, 

 I am justified in calling it chloritoid in disregard to the 

 percentage of chlorite it contains. 



Amphibole. In these rocks amphibole constantly accom- 

 panies, but is subordinate to, the mica. As is the case with 

 the latter, its crystalline planes are rarely observable. It is 

 ragged, eroded, and seems in places to have suffered partial 

 refusion at the edges and corners after crystallisation; Twin- 

 ning is not very common, but when occurring is according 

 to the ordinary law. The prismatic cleavage is well marked 

 in sections across " c." Measurements of the obtuse angles 

 in several sections gave me 124° 30', 125° 10', and 126° 15'. 

 The colours, as seen by ordinary light, are shades of green. 



The polychroism of this amphibole is well marked. I 

 found it to vary through shades of yellow and green, and 

 in some cases brown and bluish green. The colours of the 

 three rays and the absorption I found to be as follows in one 

 of the most typical sections : — 



c > b > a > 



dark green light green yellow. 



In sections which were probably near the clinopinacoid, 

 I found the angle formed by the plane of vibration to be as 



* A slight difference would be made if that portion of the felspar which is 

 kaolinised were calculated out. This is so trifling as to be immaterial. 



f Dana, System of Mineralogy, with Appendices to 1882 ; Eammelsberg, 

 Mineralchemie, 1875 ; Heddle, Mineralogical Magazine, July, 1880. 



